Twelve percent of the continental U.S. coastline may be particularly vulnerable to ocean pollution carried by freshwater seeping through the ground, according to an analysis published last week in Science.

The study provides the first detailed, continent-scale map of the underground drainage systems that link freshwater and saltwater. This underground flow of water is known as submarine groundwater discharge (SGD). That sounds abstract, but in fact it’s a familiar phenomenon: the ‘cold spots’ that people feel while swimming in the ocean are a result of this process.

“Freshwater flows out to sea, and vice versa,” says study leader Audrey Sawyer, assistant professor of earth sciences at the Ohio State University in Columbus. “Urbanization, agricultural development, climate, and topography all affect how much water flows in either direction, and the exchange has a big impact on both onshore groundwater that we drink and offshore seawater where we swim and fish.”

Scientists have struggled to get a handle on how much water enters the sea this way and where, because they can only take measurements in accessible spots, such as sandy-bottomed areas common along the Atlantic coast of the U.S. In the past, researchers have extrapolated from scattered point measurements of SGD, but this method isn’t very accurate, the new results show.

In the study, Sawyer worked with scientists from the Jet Propulsion Laboratory in Pasadena, California, to analyze large databases of information on precipitation, evaporation rates, and river flows. This enabled them to calculate how much ‘missing’ water must enter the sea underground rather than as surface runoff. Then, they used topography and land use data to identify where that water would reach the ocean.

According to the analysis, 15 billion tons of freshwater flow underground into the sea off the continental U.S. each year. That’s only 1 to 2 percent of the total flow, but SGD can carry a disproportionate amount of pollutants, as well as nutrients that contribute to harmful algal blooms and oxygen-poor ‘dead zones’ in the ocean.

Underground water flow happens everywhere along the coastline, but the amount varies greatly from place to place because of variations in topography and the locations of river courses. Just 14 percent of the coastline accounts for half of all SGD, the researchers found.

Regions that have both above-average rates of SGD and above-average levels of development are at particular risk of ocean pollution from these underground flows, as the water filters past septic tanks, fertilizer-laden agricultural fields, and contaminated industrial zones. These vulnerable regions account for 12 percent of the U.S. coastline, including the northern Gulf Coast from Mississippi to the Florida panhandle, the northern Atlantic coast, and the Pacific Northwest.

The researchers’ map also shows that 9 percent of the coastline is at risk of saltwater flowing underground toward land to contaminate freshwater aquifers. These areas include southern California, southeast Florida, and Long Island, New York. Urban development tends to decrease SGD, the researchers add, so more areas will be vulnerable to saltwater intrusion in the future.

And some areas are vulnerable to both saltwater intrusion of groundwater and coastal contamination from freshwater discharge—notably San Francisco and Los Angeles, California.

Worldwide, most of the population lives near coastlines and depends on coastal waters, so quantifying SGD and identifying contamination risk is a pressing concern. “Right now, we’ve created a map of American coastlines, but we hope to be able to do it for the world shortly, as data become available,” Sawyer says. —Sarah DeWeerdt | 9 August 2016